The invention relates to a semiconductor laser device for generating radiation beams which are substantially parallel to each other, comprising a semiconductor wafer having a first and a second major surface and comprising, between said major surfaces and beside each other, at least a first part and an adjoining second part, which semiconductor wafer is further bounded by two substantially parallel reflective side faces extending perpendicularly to the direction of said radiation beams and comprises a substrate of a first conductivity type adjoining the second major surface, on which substrate are provided successively a first passive layer of the first conductivity type, a first active layer and a second passive layer. These layers, together with the substrate, extend in both parts of the semiconductor wafer, and only in the second part there are successively provided on the second passive layer a second active layer and a third passive layer, each active layer being provided between passive layers having a larger forbidden band gap. The first active layer comprises a pn-junction in said first part and the second active layer comprises a pn-junction in said second part, each of which pn-junctions emits, at sufficiently high forward current one of said radiation beams. In the device, the substrate is connected to a first electrode, the second passive layer is connected to a second electrode, and the third passive layer is connected to a third electrode.
The invention furthermore relates to a method of manufacturing the device.
It is to be noted that when an active layer is said to comprise a p-n junction, this pn-junction may be present either inside the active layer or at the interface between the active layer and an adjoining passive layer. Furthermore, substantially parallel radiation beams are to be understood to mean radiation beams the center lines of which extend substantially parallel to each other but which in themselves may be more or less diverging.
A semiconductor laser device as described above is known from Applied Physics Letters, vol. 35, No. 8, Oct. 15, 1971, pp. 588-589.
For various application, it is of importance to have two mutually substantially parallel laser beams at a small distance from each other.
A first known application is found in devices for optical communication where light signals of a laser source are coupled into an optical fiber for transmitting information which is read out at the other end of the optical fiber by means of a radiation detector. The quantity of information (for example the number of telephone calls) which can be transported simultaneously through the same optical fiber can be doubled by using a radiation source which transmits two or more different frequencies; this is known as wavelength multiplexing. For that purpose, for example, the light of each of two lasers of different wavelengths may be coupled into a separate optical fiber and the light of the two fibers may then be combined in one single optical fiber by means of a mixing device. However, this mixing process results in considerable losses. If it were possible to couple the light of the two lasers directly in one single optical fiber, these losses could be avoided. However, this can be done only when the radiation sources are situated very close together.
Another application may be found in providing information on disks by optical methods ("digital optical recording" or DOR), in which method holes are burned in a reflecting layer by means of a laser beam. In order to check the correctness of the information thus written, it is read out by means of a second laser which is mounted behind the first laser. The two radiation beams may have the same frequencies, although for reasons of circuitry for a good separation of the signal it is desirable that that beams have different frequencies. This may be done by means of a second laser mounted in a separate optical mounting (or "light pencil") which solution, however, involves an expensive construction. The radiation beam of one single laser can also be split by means of an optical system into a "write" beam and a "read" beam. However, this is not economical since a large energy input is required for "writing" and by splitting the radiation beam the power of the laser must be increased even more, which involves problems with repsect to cooling and cost. Finally, two separate lasers mounted on one single cooling plate may also be used. For this purpose, however, the lasers must be aligned very precisely relative to each other, and the minimum distance between the emissive facets is at least equal to the width of each of the semiconductor wafers.
The semiconductor laser device described in Applied Physics Letters, 35 (8), Oct. 15, 1979, pp. 588-589 comprises in the same semiconductor body two lasers beside each other of the so-called double hetero junction type (DH lasers) with mutually parallel-extending radiating p-n junctions. However, for use in the above-mentioned applications this device has various disadvantages.
First of all, the series resistance of at least one of the integrated lasers is high since the current through said laser must flow through one of the passive layers over a comparatively large distance.
Furthermore, as is also noted in the article, the emissive facets are so far apart that it is difficult to couple the two laser beams directly in one optical fiber. This disadvantage applies to an even greater extent when said optical fiber is a so-called "monomode" fiber such as is often desired particularly in optical communication, since said fibers have very small diameters.